diff --git a/kernel/consteval.h b/kernel/consteval.h
index 844120ef0..adcf86f8a 100644
--- a/kernel/consteval.h
+++ b/kernel/consteval.h
@@ -349,7 +349,11 @@ struct ConstEval
 				return false;
 
 			bool eval_err = false;
-			RTLIL::Const eval_ret = CellTypes::eval(cell, sig_a.as_const(), sig_b.as_const(), sig_c.as_const(), sig_d.as_const(), &eval_err);
+			RTLIL::Const eval_ret;
+			if (sig_s.size() > 0 && eval(sig_s, undef, cell)) {
+				eval_ret = CellTypes::eval(cell, sig_a.as_const(), sig_b.as_const(), sig_s.as_const(), &eval_err);
+			} else
+				eval_ret = CellTypes::eval(cell, sig_a.as_const(), sig_b.as_const(), sig_c.as_const(), sig_d.as_const(), &eval_err);
 
 			if (eval_err)
 				return false;
diff --git a/passes/tests/test_cell.cc b/passes/tests/test_cell.cc
index 3a7e413ee..068386b49 100644
--- a/passes/tests/test_cell.cc
+++ b/passes/tests/test_cell.cc
@@ -1044,9 +1044,7 @@ struct TestCellPass : public Pass {
 		// $pmux doesn't work in sat, and is not supported with 'techmap -assert'
 		if (nosat && techmap_cmd.compare("aigmap") == 0)
 			cell_types[ID($pmux)] = "*";
-		// $bwmux is not supported by ConstEval
-		if (noeval)
-			cell_types[ID($bwmux)] = "A";
+		cell_types[ID($bwmux)] = "A";
 
 		cell_types[ID($slice)] = "A";
 		cell_types[ID($concat)] = "AB";